Dyoel, the item in the photo is a water/oil heat exchanger (AKA oil cooler)common to a host of VW/Audi motors. They are highly efficient. Coolant from the cold side of the radiator runs through the two small 'tubes.' The 'box' is sandwiched between the block and the oil filter. The oil circulates through the heat exchanger from the engine and back to the pan.
On another issue.... IME, the oil may not get up to 100C when the ambient temp is down in the 40s/50s on short(er) drives. I've checked my oil temp in my 1.8T Beetle after a 7-8 mile run to town; some stop/go; short stops; and a run home at 55 - 65+ mph and the oil temp in the pan is about 145F. I think the radiator so effective so it really puts out some 'cold' coolant on the cool side in cold weather. The heat exchanger is really efficient as well, so it too will suck a lot of heat out of the oil.
ARB: The problem, as you've found out, is the PCV valve. They can wear out and that's the most likely cause of 'custard' on the oil cap.
ARB stick with the 5W-40 A2/A3 or a 5W-30 A5 oil. Yes, VW recommends 5W-40 but the new ACEA standard A5 and VW standards should cover the change to a lower viscosity.